The present invention relates to a clamp for a jet pump riser in a nuclear reactor pressure vessel and to a method for arranging a clamp over a jet pump riser in a nuclear reactor pressure vessel. More specifically the present invention relates to a clamp as described which holds the pipes together in a jet pump riser in case it would crack.
A nuclear reactor has a reactor pressure vessel, which typically has a generally cylindrical shape and is closed in both ends, e.g., by a bottom head and a removable top head, to form a closed container.
A core plate and a top guide are arranged inside the reactor pressure vessel with the top guide arranged on top of the core plate. The top guide comprises a number of holes. A reactor core in the form of fuel bundles is arranged inside the reactor pressure vessel with each fuel bundle being arranged through a hole in the top guide and resting on the core plate. Some reactors are provided with a core shroud which surrounds the core, the core plate and the top guide.
The reactor pressure vessel is filled with water covering the core and the shroud. The water in the reactor pressure vessel is arranged to flow through the core in order to cool the core during operation of the nuclear reactor. The flow is usually induced by circulation pumps arranged between the shroud and the nuclear reactor pressure vessel, which make the water flow downwards in the space between the shroud and the reactor pressure vessel (RPV) wall. The circulation pumps may be of any of a number of different types. One type of circulation pumps used in nuclear reactors of the above described type is jet pumps, in which water is injected through a narrow nozzle in a jet pump arranged between the shroud and the reactor pressure vessel. Water enters the RPV through an inlet nozzle in the RPV wall. A jet pump riser assembly is connected to the inlet nozzle, wherein the jet pump riser assembly comprises a thermal sleeve, a lower elbow, a riser pipe and an upper elbow. One end of the thermal sleeve is arranged on the outside of the RPV while the other end of the thermal sleeve is welded to a first end of the lower elbow. The second end of the lower elbow is welded to a first end of the riser pipe. The second end of the riser pipe is welded to the upper elbow. During operation of the nuclear reactor a substantial amount of water is forced through the jet pump riser assembly. The large amount of water flowing through the jet pump riser assembly results in significant hydraulic forces and vibrations exerted on the jet pump assembly. Further, a large amount of heat is generated by the core during operation of the nuclear reactor, resulting in thermal stresses being exerted on the jet pump assembly. The hydraulic forces, vibration stresses and thermal stresses might cause stress corrosion cracking (SCC) in the riser pipe elbow, which might result in the riser pipe elbow being separated from the thermal sleeve. This is of course highly undesirable.
It is desirable to provide an apparatus that provides support for a jet pump riser elbow. Such an apparatus is described in U.S. Pat. No. 6,053,652, which is directed towards a clamp assembly that provides support for the riser pipe and the lower elbow. The clamp assembly comprises a lower thermal sleeve clamp that is arranged to be secured to the thermal sleeve, and a riser clamp that is configured to be secured to the riser pipe. The lower clamp and the riser clamp are coupled together. With such a clamp assembly the stresses on the weld between the thermal sleeve and the lower elbow, as well as the weld between the lower elbow and the riser pipe, are greatly reduced. This minimizes the risk for SCC of the riser pipe assembly at the interface between the lower elbow and the riser pipe, and at the interface between the lower elbow and the thermal sleeve. With a clamp assembly according to U.S. Pat. No. 6,053,652 it may however be more difficult to inspect the weld between the lower elbow and the riser pipe, and the weld between the lower elbow and the thermal sleeve.
U.S. Pat. No. 6,108,391 describes a clamp apparatus for performing jet pump riser pipe repairs. The clamp apparatus described in the '391 patent includes a lower clamp element and an upper clamp element. The upper and lower elements are configured to be positioned at the interface between the thermal sleeve and the jet pump riser elbow. The upper and lower elements include extended ridges that are configured to fit in circumferential grooves precisely machined into the sleeve-elbow assembly on opposing sides of the interface.
One disadvantage of the clamp apparatus described in the '391 patent is that it requires grooves to be precisely machined into the sleeve-elbow assembly on opposing sides of the interface.
The apparatuses according to the prior art may be used to support the interfaces between the riser pipe, the lower elbow and the thermal sleeve in a riser pipe assembly in a reactor pressure vessel. However, there is a need for an alternative clamp apparatus that solves at least one of the problems of the prior art.